In recent years, ceramics, ceramic matrix composites and metal matrix composites have gained increasing acceptance in the industry. More specifically, such materials are attractive due to, for example, increased wear resistance, as opposed to, for example, metals. However, in attempting to incorporate ceramics, ceramic matrix composites and metal matrix composites into applications previously served by metals, complications may arise.
For example, ceramics, ceramic matrix composites and metal matrix composites can be manufactured to have relatively high hardness values. However, such high hardness values may make such materials difficult and expensive to machine. Therefore, as described herein, in certain instances when the end application permits, it may be desirable to provide the ceramic, ceramic matrix composite or metal matrix composite with a polymer-based coating on at least one surface thereof, wherein the polymer-based material is manufactured to be readily machineable and provides an attractive finish. Moreover, certain ceramic, ceramic matrix composite and metal matrix composite materials may be too brittle for certain applications and thus could be susceptible to, for example, chipping or cracking. Therefore, as discussed herein, it may be desirable to provide such materials with a protective polymer-based coating in order to provide the material with suitable protection from chipping or cracking due to, for example, shipping or handling misuse. Furthermore, certain end use applications result in undesirable stresses being applied to such materials. For example, the ceramic, ceramic matrix composite or metal matrix composite may be used in combination with (e.g., coupled) with a material having a substantially different coefficient of thermal expansion (CTE) which can result in the generation of undesirable thermal stresses. Therefore, it may be desirable to provide the ceramic, ceramic matrix composite, or metal matrix composite with a polymer-based material between at least a portion of the interface between the two materials having a different CTE. The interface material can be designed to absorb any stresses realized when two coupled materials expand at differing rates.
One specific application which benefits from the materials of the instant invention are shaft sleeves. Shaft sleeves are used with centrifugal slurry pumps in a part of the pump called a stuffing box. The stuffing box is used to separate the wet end of the pump (i.e., the actual pumping mechanism) from the dry end of the pump (i.e., the gear box and motor). The shaft sleeves of the prior art are generally cylinders of metal that fit tightly onto and around the shaft, which are also generally metal or steel, that connects the motor to the pump end impeller and rotates with the shaft.
Packing material, usually made of, for example, carbon impregnated Teflon.RTM. cord is placed around the shaft sleeve and held tightly against it by the steel casing of the stuffing box. The packing is stationary and does not rotate. It is generally desirable to pack the packing material as tightly as possible against the rotating shaft sleeve, since this tight packing will limit the amount of leakage of liquid from the wet end of the pump into the dry end of the pump. However, this creates a significant amount of friction which damages the packing material and wears out the shaft sleeve due to, for example, abrasion. Moreover, when pumping abrasive slurries, the situation is further aggravated because even small leaks of an abrasive slurry may increase the wear on both the packing and the shaft sleeve.
As stated above, the shaft and shaft sleeves of the prior art are generally made of a metal (e.g., steel) and may not be particularly well suited for an erosive and/or corrosive environment which can be generated in a stuffing box of a centrifugal slurry pump. In an effort to promote the life of the shaft sleeves and consequently the shaft of centrifugal pumps, recent attention has been directed to the use of ceramics as shaft sleeve materials due to the advantageous properties of ceramic materials. However, as is well known to those skilled in the art, most metals which can be utilized as a shaft have CTEs which, typically, differ quite significantly from CTEs of most ceramic materials which could be utilized as a shaft sleeve. Therefore, when a metal shaft/ceramic shaft sleeve combination is subjected to friction, which results in the generation of heat in both the metal shaft and ceramic shaft sleeve, the metal or steel shaft tends to expand at a greater rate than the ceramic shaft sleeve resulting in the generation of tensile stresses in the ceramic shaft sleeve, which stresses may cause the ceramic shaft sleeve to crack and/or fail catastrophically.
Another specific example of an application which would benefit from the combination of materials of the instant invention would be hydrocyclone separators. More specifically, the apex of a hydrocyclone separator is subjected to a highly abrasive and/or corrosive environment. Therefore, it may be desirable to utilize a ceramic, ceramic matrix composite or metal matrix composite as at least a portion of an apex because such materials can be manufactured to withstand such an abrasive and/or corrosive environment. However, while a ceramic, ceramic matrix composite or metal matrix composite apex may be well suited for the intended purpose, the outer surface of the apex may be easily damaged during, for example, installation or shipping.
Furthermore, an apex is generally inserted into a steel housing and attached thereto utilizing, for example, an epoxy and a mechanical clamping ring. Accordingly, a ceramic, ceramic matrix composite or metal matrix composite may also be benefited by providing a polymer-based coating on at least a portion of the outer surface thereof.
The present invention provides novel materials which overcome all of the above difficulties. Particularly, for some preferred embodiments of the invention, attractive ceramic, ceramic matrix composite and metal matrix composite materials for use in the present invention have been developed by the assignee of the current application and are discussed herein below.